Is there a natural remedy to fight ravages of the processionary caterpillar on the Atlas cedar?

Several factors put pressure on the cedars, resulting in the reduction of forest areas, the degradation of biodiversity and the Atlas cedar’s deterioration. The processionary caterpillar is a “pest” that weakens the health of the cedar. Is a “natural” solution to this plague possible?

It is a small caterpillar, but as small as it is, it threatens the health of the multi-millennial cedars. Its name is the processionary caterpillar. The decline of cedars in the Middle Atlas is not a new matter. It is a complex phenomenon whose causes are sometimes very diverse, difficult to identify as to prioritize.

Many factors play an active role in the degradation of Moroccan forests, especially the cedar forests. Anthropic pressures (generated by human) is undoubtedly the most devastating. Among these factors, “pest” insects can also be involved in the dieback process.

There are three factors implicated in a particular chronological order. The first one is called “predisposing factor” (performed by phyllophagous insects), the second one is called “triggering factor” (performed by phyllophagous and xylophagous insects) and the third one is called “aggravating factor” (performed by xylophagous insects). Insects therefore always intervene at one point or another in the evolution of this phenomenon.

There are three classes of insects:

Phyllophagous that feed on needles and cedar sap. Among them are lepidopterans (processionary caterpillars, Bombyx), beetles, hemiptera (mealybugs) and homoptera (aphids).

The xylophagous that constitute the main insect attacking cedar wood including beetles (bark beetles, longhorn beetles…).

Some insects such as lepidopterans (borer) and diptera like to eat the cones and seeds of the tree.

Processionary caterpillars that can attack cedar are Thaumetopoea bonjeani Powell, and Thaumetopoea pityocampa Schiff. All can spread throughout the tree regardless of age and location, attacking either new foliage or old.

As a result, cedars can be totally defoliated by these phytophagous pests, leading to their weakening and predisposing them to the attacks of xylophagous insect pests.

The processionary larva (Thaumetopoea) is a disaster and wreaks havoc on Atlas cedar and Maritime Pine.

Where does this caterpillar come from though? After field studies of our group of researchers, we found that this pest most likely appeared because of the introduction of pine in the cedar forest.

Since the last century, forestry workers have sought and tested many treatments to eliminate it from our landscape. They started with chemical treatments. After a while, they – in partnership with the scientists – noticed serious impacts on conifers as well as on fauna and micro-fauna. It was therefore necessary to look for another method.

Scolytus numidicus

Callidium cedri

The method currently used to fight this caterpillar is microbiological control.

Carried out from the ground, however, it can be limited in its effectiveness by constraints related to the active ingredient or its spreading means.

The material used is called Bacillus Thuringia, Bacillus thuringiensis ssp. kurstaki, commonly referred to by its acronym BTK. It is a bacterium that lives naturally in the soil. In the last 30 years, forest workers have been using it worldwide as a biological control agent to suppress populations of various forest and agricultural insect pests.

This bacterium is considered nontoxic to humans and mammals. Treatments take place in the fall to spare other beneficial insects, as many butterflies are no longer in the larval stage. Bacillus thuringiensis acts on the digestive system of caterpillars when they ingest the active ingredient at the same time as the pine needles.

Bacillus persists 8 to 10 days in good conditions.

Its effectiveness in limiting the population of caterpillars is therefore conditioned by its ingestion in this period.

However, this treatment of the processionary caterpillar has limitations. Carried out too late, it will increase the damage on the plants and the risks in terms of public or animal health.

Is a “natural” treatment possible?

The only way protecting biodiversity and eliminating the processionary caterpillar at the same time is through biological control by predation: this ecological pest consists in encouraging the natural regulation of a species by its predators.

This local predator must meet criteria, respecting the ecological balance. This introduction should not affect other species.

A team of researchers from the Sidi Mohamed Ben Abdellah University of Fez is currently working on a treatment of this kind. The objective is to limit the processionary caterpillars. The idea is to favor the nesting of a local bird by setting up specific nesting boxes, in order to increase the population and the predation pressure applied on the caterpillars.

This local bird should be insensitive to the urticating hairs of the caterpillar. Nesting boxes must be installed before the nesting period (early spring) and meet some criteria to encourage these “caterpillar predators” to settle there as height, orientation or protection against its own predators. The team is currently working on these parameters. If the “natural” solution were successful, it would be a first of its kind.

“There is no fruit that has no worm, no flower that has no caterpillar, no pleasure that has no pain: our happiness is only a misfortune more or less consoled” once wrote Jean-François Ducis…